Distribution of gold and some base metals in the Slana area, eastern Alaska Range, Alaska

.};'(if!:)fif.'.:~::::..';·;;)(:;:;:{·) Work done in cooperation with the State of Alaska, Division of Mines and Minerals

GEOLOGICAL SURVEY CIRCULAR

Work done in cooperation with

the State of Alaska, Division

of Mines and Minerals

Geological Survey

Abstract- - - - - - Introduction - - - - Geologic setting- - - - - - - Economic geology- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Geochemistry- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Sampling and analytical procedures - - - - - - - - - - - - - - - - - - - Background and anomalous concentrations of metals - - - - - - - - - - Distribution of metals in stream sediments - - - - - - - - - - - - - - - Distribution of gold in rocks - - - - - - - - - - - - - - - - - - - - - - - Economic potential - - - - - - - - - - - - - - - - - - - - - - - - - - - - - References cited - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

4-8. Maps showing distribution of gold and base metals in the

2. Gold content of rocks- - - - - - - - - - - - - - - - - - - - - - -
3. Concentration of gold and base metals- - - - - - - - - - - - - -
4. Index map of part of south-central Alaska showing
5. Sample-location map, Slana area- - - - - - - - - - - - - - -
6. Explanation for geologic maps showing distribution

Slana area- - - - - - - - - - - - - - - - - - - - - - - - - -

of gold and base metals (figs. 4 -8)- - - - - - - - - - - - -

Slana area:

4. Gold - - - - - - - - - - - - - - - - - - - - - - - - - -
5. Copper- - - - - - - - - - - - - - - - - - - - - - - - - -
6. Lead - - - - - - - - - - - - - - - - - - - - - - - - - - -
7. Zinc - - - - - - - - - - - - - - - - - - - - - - - - - - -
8. Molybdenum- - - - - - - - - - - - - - - - - - - - - - - CONTENTS

By Donald H. Richter and Neal A. Matson, Jr.

The Alaska Division of Mines and Minerals made geologic and geochemical studies of the Slana area of the eastern Alaska Range from 1963 to 1966. The s e studies resulted in a geologic report (Richter, 1966) and in the identification of two base-metal anomalies (Richter, 1~65). The investigations were continued and extended by the U.S. Geological Survey in 1967 as part of its Heavy Metals program particularly to determine the relationship of gold to the geologic setting. Gold analyses made on all the stream-sediment samples and many rock samples collected during the Statesponsored studies disclosed sev~eral gold anomalies and the fact that the geologic associations of gold are markedly different from those of the base metals. This report presents all the newly available data on gold distribution as related to geology of the Slana are a together with a review of the published data on the base metals.

The Slana area, as informally r e fer r e d to here, covers approximately 240 square miles on the south flank of the eastern Alaska Range, mostly between the Chistochina and Slana Rivers (fig. 1). It includes parts of the Gu1kana C-1 and D-1 and the Nabesna C-6 and D-6 quadrangle maps (scale 1:63,360). The Tok Cutoff of the Glenn Highway runs about 26 miles through the area and affords relatively easy access to most of the back country. GEOLOGIC SETTING The Slana area lies south of the Den a 1 i fault, a major trans-Alaska strike-slip lineament, in aregion of upper Paleozoic and Mesozoic sedimentary and volcanic rocks that have been intruded by two compositionally and texturally distinct groups of igneous rocks. A genera'Iized stratigraphic column and brief description of the bedded rocks are given below.

A r g i 11 i t e containing Cretaceous and Jurassic

Angular unconformity. Amygdaloidal b a sa 1 t Permian - - - - - - - -

Unconformity. Permian - - - - - - - - - - - - Limestone, argillite,

Permian - - - - - - - - - - - - Volcaniclastic r o c k s

Permian and Pennsylvanian(?) - - - - - - - Andesitic v o 1can i c

T hi s sedimentary and volcanic sequence, except for some of the younger Mesozoic rocks, was intruded by a large heterogeneous diorite-quartz diorite complex and a number of satellitic( ?) bod i e s whose composition was similar but also inc 1 u de d granodiorite, gabbro, and anorthosite. A more alkalic, zoned pluton of quartz monzonite and granodiorite, Description

i n t e r b e d d e d siltstone, g r a y w a c k e, and conglomerate.

containing interbedded limestone, bas a 1 t, and limestone conglomerate.

and minor coarser grained clastic r o c k s (Manko men? Formation).

c on t a i n i n g minor imp u r e limestone.

rocks, including flows, tuffs and breccias, mud-avalanche deposits, and volcaniclastic rocks. termed the Ahtell Creek pluton in previous reports, intruded the Permian volcanic and volcaniclastic rocks in the western part of the area. Both groups of intrusive igneous rocks are younger than Permian and probably older that Late Jurassic, although neither their absolute nor relative age is well known.

The structural grain of the area, revealed by faults, fold axes, bedding, and orientation of some of the dioritic intrusives, parallels the northwestward trend of the Alaska Range and its tectonic elements. Only the Ahtell Creek pluton, which trends northward, appears to be discordant with the regional structure. Bedding and flow layering in most of the rocks dip at moderate angles to the northeast toward the Denali fault.

ECONOMIC GEOLOGY Prospecting has been carried out in the Slana area since before the turn of the century, and between then and the early 1930's a number of argentiferous base-metal quartz veins were discovered. Placer gold was also discovered during the early prospecting. Figures 4-8 show the distribution of gold and base metals in the area. The letters and numbers used with references to figures in subsequent sections are map coordinates.

The veins and the lead, zinc, and m o I y b den u m anomalies are spatially and apparently genetically r e 1ate d to the Ahtell Creek quartz monzonitegranodiorite pluton. These veins and anomalies indicative of possible mineral deposits occur chiefly in the fine-grained border zone around the southern 1o be of the pluton and in adjacent hornfels country rock. Most of the veins are thin simple hydrothermal fissure fillings which are dominantly massive white quartz and locally minor barite and carbonate minerals and scattered crystals or segregations of galena, chalcopyrite, sphalerite, andargentiferous tetrahedrite. In the strongly altered area south of Long Lake (figs. 4-8, C-2), small scattered flakes of molybdenite are associated with quartz, pyrite, and sericite.

The placer gold in Grubstake and Slope Creeks is fine grained and wiry and shows no evidence of lengthy transport. Earlier it had been thought that the gold was also derived from lode deposits within the border zone of the Ahtell Creek pluton (Richter, 1965 ); however, the data presented here suggest that the gold is not associated with this pluton but rather with the more mafic diorite-quartz diorite intrusives.

GEOCHEMISTRY The geochemical data presented in this report are based on the chemical and spectrographic analyses of 258 samples of stream sediments and 105 samples of rock collected between 1963 and 1967. These data show anomalous concentrations of gold and base metals in many stream sediments throughout the Slana area and define a number of anomalies which may be significant. Emphasis is directed toward the gold anomalies and the distribution of gold in the country rock; the base-metal an om a 1 i e s have been discussed in earlier reports (Richter, 1965, 1966), and only a brief review of their salient features is presented here.

Sample locations for both stream sediments and rocks are shown in figure 2. Analytical results are given in tables 1 and 2 and are shown on the metaldistribution maps (figs. 4-8). Sampling and Analytical Procedures Stream-sediment samples were collected from active streams at sites yielding as much fine material as possible. Copper, lead, zinc, and molybdenum analyses were made on the -80-mesh fraction of all samples by a number of laboratories using colorimetric, atomic-absorption, or spectrographic techniques (see headnote, table 1). Gold analyses were also made on the -80-mesh material by Geological Survey laboratories using a hydrobromic acid extraction and atomic-absorption spectrophotometry. For samples weighing 10 grams or more the lower limit of determination for gold was 0.02 ppm (parts per million). However, many samples weighed less than 10 grams, and for these, 5-, 2-, or even 1-gram amounts were used; these small samples raised the lower limit of determination to 0.04, 0.1, and 0.2 ppm gold, respectively. Hence, not all gold values are directly comparable, and it is very probable that many of the small samples contain gold in concentrations exceeding 0.02 ppm.

The 105 rock samples analyzed for gold were selected from available specimens in an attempt to insure a representative suite of the various rock types and to give as much geographic coverage as possible. The samples were crushed, ground to -150 mesh, and analyzed by the same method used for the stream sediments. Ten grams of material was available for all the rock analyses. Background and Anomalous Concentrations ofMetals Data on the concentration of gold, copper, lead, zinc, and molybdenum in the rocks and stream sediments of the Slana area are summarized in table 3 and compared with average crustal abundances of these elements.

These data indicate that the Slana area as a whole is not anomalous with respect to any of the analyzed elements. All the elements-with the possible exception of gold, for which data below a concentration of 0.02 ppm are not available-generally occur in concen- · trations of the same order of magnitude as their estimated abundance in the earth's continental crust. Moreover, both the mode (most frequent concentration) calculated from stream-sediment samples and the mean and mode calculated from rock samples for the four base metals are virtually the same. The apparent low mode for copper in rocks may be due to limited rock-sample data.

In this report, concentrations of copper, lead, zinc, and molybdenum approximately t h r e e times mean background or more are considered anomalous; these concentrations are for copper, 150 ppm; lead, 30ppm; zinc, 180 ppm; and molybdenum, 6 ppm. These values correspond rough 1 y to the break point on the lognormal frequency distribution curves of the met a 1 s in stream sediments of the Slana area (Richter, 1966 ). For gold the lower limit of detection, or 0.02 ppm, is considered anomalous.

Distribution of Metals in Stream Sediments The two conspicuous base-metal anomalies in the area, mentioned and described previously (Richter, 1965, 1966), are obvious from inspection of the metal distribution maps. The 1 a r g est of these is a leadmolybdenum anomaly in the vicinity of Long La k e (figs. 6 and 8, C-1, -2, andD-1, -2). Slightly anomalous zinc con cent r at ion s were detected in a few streams, but with the exception of one stream, copper was not present in amounts much above background. The other anomaly, high in lead and zinc, is south of Flat Creek (figs. 6 and 7, E-2 and F -2). Both anomalies are largely within the border zone of the Ahtell Creek pluton. Conspicuous quartz-pyrite alteration zones, some as large as 5 acres in extent, and a few thin quartz-sulfide veins are exposed near Long Lake. The Flat Creek anomaly, which is currently being explored by private industry, is in an area of low relief and extensive tundra cover, and no mineralized outcrops were found.

West of Ahtell Creek a number of streams draining the border zone of the Ahtell Creek pluton contain slightly anomalous amount s of one or more base metals. The relatively high concentration of lead in E-4 (fig. 6) is on a small stream draining the Silver Shield silver-lead prospect.

Copper is rather widely distributed throughout the area and shows a closer spatial association with the diorite-quartz diorite intrusives than with the Ahtell Creek pluton. Anomalous concentrations of copper occur sporadically along the entire southwest flank of the large diorite-quartz diorite mass and 1 o c a 11 y around a number of the smaller dioritic intrusives (fig. 5). The strongest anomaly, which shows concentrations as high as 400 ppm, is southwest of Indian Pass Lake (fig. 5, C-4, -5 and D-4) in an area of numerous small hornblende-diorite intrusives. Another small hornblende-diorite intrusive in B -2 and C-3 is also the apparent source of a smaller anomaly. The hornblende diorites in both these anomalous areas are locally pyritized and conspicuously limonite stained. The single copper anomaly in the northeast part of the area (fig. 5, A-9) is on a stream draining cupriferous amygdaloidal basalts and is the only anomaly not known to be related to intrusive rocks.

The distribution of gold in the Slana area, as defined by the analyses of stream sediments, shows, like cop p e r, a marked association with the diorite intrusives and with a few exceptions does not correspond to the distribution of the other base met a l s. Of more than 30 streams with detectable gold, only five drain the quartz-vein-rich border zone of the Ahtell Creek pluton and its peripheral hornfels zone. Three of these, including one with the strongest stream-sediment anomaly detected (7 .1 ppm gold), are in an area where dioritic rocks also occur (fig. 4, F-2).

The strongest and most conspicuous gold anomaly is southwest of Indian Pass Lake (fig. 4, C-4, -5 and D-4) in an irregular area underlain by a number of small hornblende diorite intrusives and is coincident with the strong copper anomaly. Sediments in 13 streams draining the area contain more than 0.02 ppm gold, and sediments in three of these contain 2 ppm gold. The local placer go 1d in Grubstake and Slope Creeks apparently also has its source in this area and not in the nearby border zone of the Ahtell Creek pluton, as previously thought. Surprisingly, the sediments in Grubstake Creek, the larger of the two placer deposits, are only moderately enriched in gold (0.04 ppm, sample 94), and this fact suggests that some other streams, which show much higher gold concentrations, could also contain gold placer deposits.

Elsewhere in the area anomalous concentrations of gold occur principally along the southwest border of the large diorite-quartz diorite complex (fig. 4, B-5 and C-6), southeast of the Slana River around an arcuate quartz diorite intrusive (fig. 4, D-8, -9 and E-8), and west of Ahtell Creek near a small diorite intrusive (fig. 4, B -3 ). These t h r e e anomalies are defined by only a few stream-sediment samples with gold concentrations generally well below 2 ppm and are not nearly as promising as the an om a 1y near Indian Pass Lake. Distribution of Gold in Rocks Gold analyses on 105 rocks collected throughout the area as representative geologic samples-not as ore mineral samples-show a golddistributionpattern similar to that shown by the stream-sediment data. Gold analyses of rocks are 1 i s ted in tab 1 e 2 and plotted on the gold distribution map (fig. 4 ).

Only 11 of the rocks analyzed contain detectable gold (0.02 ppm or more), and all these are diorites, quartz diorites, or hornblende-feldspar porphyries believed to be hypabyssal equivalents of the dioritic rocks. Moreover, none of the rocks containing detectable gold is from an area outside the anomalies defined by the stream sediments. In particular, the anomaly southwest of Indian Pass Lake includes the most rock samples (seven) showing enrichment in gold 2:.nd also the sample with the highest gold content (0.3 ppm, sample R60). Two diorite sa mp 1 e s from the anomalous area in B-5 (fig. 4) are enriched in gold, as are two quartz diorite samples from the anomalous area southeast of the Slana River. Many of the rocks containing detectable gold also con t a in disseminated pyrite, but pyrite is common in intrusive rocks in the area and probably is equally or m or e abundant in some of the analyzed quartz monzonitegranodiorite rocks and hornfels.

ECONOMIC POTENTIAL The gold geochemical anomaly in the central part of the Slana area appears to warrant further exploration. Sediments in at least three streams draining the anomalous area contain in excess of 2 ppm gold, a fact suggesting the presence of local placer deposits. However, it is probable that the original lode gold (possibly in quartz veins) was coarse enough to supply material for workable placer deposits only in the restricted area in the border zone of the Ahtell Creek pluton at the head of Grubstake and Slope Creeks. lf, as seems likely, the gold is disseminated throughout the diorites, gold particles in the other streams draining the anomalous area may be extremely fine grained and not recoverable by conventional placer mining.

The possibility of a large low-grade lode deposit within the apparent dioritic source rocks of the anomaly should not be discounted. Detailed sampling of the dioritic intrusives, especially the more pyrite-rich areas, rm y reveal gold concentrations higher than the 0.3 ppm found in this reconnaissance sampling of the bedrock.

Diorite and quartz diorite intrusives are not restricted to the Slana area of the eastern Alaska Range but are known to occur throughout the range from west-

areas these dioritic rocks are conspicuously altered and (or) contain abundant disseminated sulfides. However, little is known concerning the gold content of these rocks. One area in which gold deposits seem to be genetically related to diorite is in the Chulitna district of the central Alaska Range, where Ross (1933) reported that some of the lodes are disseminated iron sulfide deposits in diorite porphyry.

REFERENCES CITED Moffit, F. H., 1938, Geology of the Slana-Tokdistrict, Alaska: U.S. Geol. Survey Bull. 904, 54 p. Richter, D. H., 1965, Geochemical investigation ofthe Slana district, south-central Alaska, 1963 and 1964: Alaska Div. Mines and Minerals Geochem. Rept. 2, 14 p. 1966, Geology of the Slana district, south- __c_e_n-tral Alaska: Alaska Div. Mines and Minerals Geol. Rept. 21, 51 p. Ross, C. P., 1933, Mineral deposits near the West Fork of the Chulitna River, Alaska: U.S. Geol. Survey Bull. 849-E, p. 289-333. Taylor, S. R., 1964, Abundance of chemical ele~ents in the continental crust-A new table: Geoch1m. et Cosmochim. Acta, v. 28, no. 8, p. 1273-1285.

so

so

so

,us

l.. s

L U N

r-- _,-- ·;_J4'~-- - ~· • •

!-(Ci_ __ ·

t·

""=- - -~=_l~==='- #-=-, tR~,1 , . 44 -H5

l

J

,, -~~-r=-=-- ---+-l, --_- -~ ~~~~3~ "I R41·R·3o .::9 ~l'tr:: '~ ~ r-- >-'~ _.-----._-~-5--~· ~------'.: ·-,-,-1-,:---- ~~-n-- I"£.~, ' """ 1 ~ l l }--~-

~1 0 I I

f I ' k ~ R~o •is ~-~~ .}_? k

"'20yl _;)>~' 5 o\207

b2 ·~~~:r~zt~7

-t----1-

Figure 2.--Sample-location map, Slana area.

.~,O! Rt :_ 2 i2~~~f,.~:~/~

1---==-t

u

u

~ Z 5

c

J

• ?aoo

Figure 5.--Map showing distribution of copper in the Slana area.

o(\ PP~

0-.tJ~~"I!i

Figure 6.--Map showing distribution of lead in the Slana area.

Figure 7.--Map showing distribution of zinc in the Slana.

~~p~, .· '.

w···~" '~

~KJa

c:P\

Where this page came from

This page was imported from U.S. Geological Survey. Published by the U.S. Geological Survey and, as a work of the United States government, in the public domain.

Nobody has written it yet — it is the source material at a new address, which is why search engines are asked to skip it and why no one earns from it. It is up for grabs: take it on, and it is yours to rewrite and to earn from.